N-type compound semiconductor crystals, particularly GaAs, AlGaAs and GaP, are used on a large scale as substrate materials for luminescent semiconductor diodes emitting visible light or near-infrared light. The active light-generating field of the diodes is manufactured on the substrate material. The rear of the substrate is normally provided with an ohmic contact. Most known ohmic contacts for n-type III-V compound semiconductor crystals are made with the aid of a so-called alloy process. The procedure characteristic for alloying is the melting of a metal film onto the semiconductor surface. The metal film generally comprises--in the case of contacts on n-type III-V compound semiconductors--a eutectic AuGe or AuGeNi alloy. According to an explanation generally accepted today for the ohmic behavior of the alloyed contacts, part of the III-V compound semiconductor crystal is dissolved in the metal melt during the alloying process, in order to crystallize during cooling and to regrow epitaxially on the III-V compound semiconductor crystal. In the new layer thereby generated, there is sufficient germanium to dope the III-V compound semiconductor crystal as the n-type high enough for the field emission to dominate at the potential threshold between the contact metal and the semiconductor crystal. To achieve an ohmic behavior of the contacts as good as that observed in alloyed AuGe and AuGeNi contacts, a doping concentration of at least 5.multidot.10.sup.19 cm.sup.-3 is required. A typical drawback of the alloyed contacts is their inhomogeneity both in the lateral and in the vertical direction in relation to the semiconductor surface. The alloying process generally results in a rough and inhomogeneous boundary surface formed by a large number of differently sized droplike or microcrystalline elevations containing germanium.
Depending on the size of the energy gap between the conduction band and the valence band of the substrate material and on the energy of the radiation generated by the luminescent semiconductor diode, the substrate material is at least partly transparent for the radiation generated. The highly germanium-doped areas that result from the alloying process of the AuGe or AuGeNi contact on the rear of the substrate material however absorb the light hitting them and thereby cause considerable absorption losses. Both the quality and the reproducibility of an alloyed contact frequently remain unsatisfactory. This applies in particular for the electrical and optical properties and to the morphology of the alloyed contacts.
The object of the present invention is therefore to provide a method for manufacturing ohmic contacts on an n-doped semiconductor layer or a III-V compound semiconductor, with the contacts having a low contact resistance and a high reflectivity for radiation in the visible spectral and near-infrared ranges.